The present invention relates to an optical attenuator and an optical attenuator module for attenuating the intensity of light by a specific amount used in optical communication system or the like.
Various devices have been proposed so far as optical attenuators used in optical communication systems or the like. Among conventional optical attenuators, an optical device of electric, magnetic or thermo-optical crystal is inserted between optical fibers, and its transmissivity is changed, or a lens is disposed between optical fibers, and a necessary attenuation is obtained by making use of the interval or axial deviation. Other optical attenuator using an attenuator plate between optical fibers has also been proposed.
In such conventional optical attenuators, however, accurate attenuation factor or stability could not be obtained. That is, the attenuator which attenuates by incident light the interval or axial deviation of two optical fibers did not have enough stability and attenuation factor of a high resolution. Or, in the constitution of inserting a device of electric, magnetic or thermo-optic crystal between optical fibers, insertion loss or dependence on plane of polarization may be involved, and it is hard to obtain a desired attenuation factor. In the type of using an attenuator plate, since the attenuation factor varies depending on the wavelength of incident light, it is impossible to obtain a specific attenuation factor regardless of wavelength.
The invention is devised in the light of the problems of the prior arts, and purposes to provide an optical attenuator capable of obtaining a desired attenuation factor, without depending on the wavelength in a specific wavelength range, and an optical attenuator module using the same.
An attenuator of the invention comprises a transparent glass substrate with a specific taper so that the incident plane and exit plane may not be parallel to each other, and a metal film formed on one side of the transparent glass substrate for attenuating the incident light by a specific attenuation factor. It hence eliminates fine ripple of attenuation factor depending on wavelength, and a desired attenuation characteristic is obtained.
Moreover, antireflection coating may be provided on at least one side of the transparent glass substrate. The antireflection coating is intended to compensate for wavelength dependence of attenuation factor due to the metal film, and it has the reverse characteristic of the dependence of the metal film on wavelength in the operating wavelength range, and the entire attenuation factor of the optical attenuator may not depend on the wavelength.
A preferred aspect of the invention is a variable type optical attenuator for attenuating the incident light so that the attenuation factor may vary depending on the incident position, which comprises a transparent glass substrate with a specific taper so that the incident plane and exit plane may not be parallel to each other, a metal film formed on one side of the transparent glass substrate for attenuating the incident light by a specific attenuation factor depending on the incident position, and an antireflection coating provided on at least one side of the transparent glass substrate for compensating for wavelength dependence of attenuation factor due to the metal film. By changing the film thickness of the metal film in a specific direction, the attenuation factor can be changed corresponding to the incident position. In this case, the wavelength dependence can be eliminated by the antireflection coating.
When such optical attenuator is combined with a first optical branching unit for branching part of incident light and sending the rest into the optical attenuator, a second optical branching unit for branching part of the light passing through the optical attenuator, and passing the rest, first and second photo detectors for receiving the lights branched by the first and second optical branching units respectively, a level detector for detecting the reception level of the first and second photo detectors, and a display unit for displaying the reception level detected by the level detector, an optical attenuator module is constituted.
Further, using an optical attenuator changing in the attenuation factor depending on the incident position, an optical attenuator module can be composed together with an incident position adjusting unit for changing the position of the light entering the optical attenuator. Thus, the attenuation factor can be controlled depending on the incident position.
Moreover, combining this optical attenuator module with an attenuation factor calculator and a feedback control unit for changing the incident position by the incident position adjusting unit so that the calculated attenuation factor may be a preset attenuation factor, it is possible to adjust to the preset attenuation factor.